Is it time for proportional representation?

Haven't we heard all this before?

Yes. Ditching Britain's old-fashioned 'first-past-the-post' electoral rules for a more modern and inclusive proportional representation system is an idea that seems to resurface whenever it might help the Government. Thus it got serious attention in 1997, when Tony Blair and Paddy Ashdown, then leader of the Lib Dems, set up a commission to investigate alternatives to first-past-the-post. The idea was that, with the Tories in disarray, a new system could help establish an almost permanent centre-left majority in Parliament. And now Gordon Brown is exploring constitutional reform as part of a desperate attempt to respond to the MPs' expenses scandal and shore up his own position. The PM said last week that he'd consider dropping first-past-the-post if there was a "broad consensus in the country" - code for a referendum.

But isn't there much to be said for the current system?

The great advantage of first-past-the-post is that it produces clear winners and losers. Since each constituency produces an outright winner, even if by a single vote, there's more chance of producing decisive election results and hence governments with a clear mandate to rule. (They can then be thrown out just as sharply.) But by the same token, first-past-the-post tends to magnify the power of the big parties. In 1983, for example, the Liberal/SDP Alliance received 25.4 per cent of the vote, a mere 2 per cent less than Labour; yet it won just 23 MPs while Labour got 209. And that points to what many see as another grave disadvantage of first-past-the-post – the fact that most of our votes are 'wasted'.

How are they supposed to be 'wasted'?

First-past-the-post doesn't care who comes second, still less third, which means that in most areas of the country with safe seats, you may as well not bother to vote. There are many Tory voters in Scotland, for example, but as they form the minority in each constituency, Scotland has just one single Tory MP; Labour suffers in the same way in much of southeast England. As a result British general elections pivot on the 200,000 or so voters in a handful of marginal constituencies who effectively decide the whole thing. Those arguing for PR cite this as a major reason for low turnouts at general elections. The way to reinvigorate British politics, they say, is by using a better voting system.

What does that system involve?

There are lots of PR systems - the Electoral Reform Society's preferred option is the Single Transferable Vote - but the aim of each is to reflect the plurality of opinion within a constituency. For this purpose constituencies tend to be much larger than those used for the House of Commons, and they return not